Single "pulse" injection of H3-thymidine have been used to define the birthdates of nerve cells in the cat dorsal lateral geniculate nucleus. Using the following procedure dividing cells destined to form the dorsal lateral geniculate nucleus have been radioactively labeled in individual kitten fetuses of known gestational age. In order to gain access to the uterus a laparotomy is performed in each mother cat. Once the uterus has been exposed a single injection of H3-thymidine (500 uCi in .05cc of sterile water) is made, through the uterine wall, into each conceptus. The intact uterus is then returned to its normal position in the abdomen and all surgical incisions closed. Following surgery the mother cat is housed in a metabolic cage for a period of two weeks during which time all waste products are collected and checked for radioactivity. At the end of the two weeks the mother cat is returned to her home cage and allowed to deliver her kittens normally. All kittens are reared for at least two months with either normal binocular visual experience or with the lids of one eye sutured closed. Each animal is then sacrificed and blocks of brain tissue embedded in paraffin and processed routinely for autoradiography. Since the radioactively labeled thymidine is injected directly into each conceptus it is necessary to expose the autoradiographs for only three-to-four weeks. The results of these injections show that most dorsal lateral geniculate nucleus cells in the cat are generated during a period of rapid cell proliferation that begins during the fourth week of gestation and continues for slightly longer than one week. Although cell proliferation is most rapid between gestational days 25 and 29, a few dorsal lateral geniculate nucleus cells are born as late as gestational day 33. Other visual system structures contain cells with birthdated laterthan gestational day 33.